Atrazine is an important agriculture herbicide for weed control in the cultivation of corn and sorghum. This experiment is to study the cooperative hydrogen bonding and hydrogen bonding potential of CF3-labeled atrazine. A nonlinear optimization procedure is developed to find the dimerization constants of atrazine from the NMR chemical shifts of four of its NH protons. The NH protons identify four active sites which interact pairwise by cooperative association to form a total of 10 hydrogen-bonded dimers. Each active site is assumed to have a partial dimerization constant related its chemical potential, and the partial constants are used to develop equations which fit the chemical shift data to a cooperative dimerization model. The partial dimerization constants are multiplied together pairwise to obtain the dimerization constant for each of the 10 atrazine dimers. We will run 19F-NMR (5mm Tube) of CF3-atrazine in CCl4 containing the following organic complexing agents at concentrations from 0.71 mM to 4.5 mM: acetic acid (HOAC), pyrrolidine (CNH), cyclopentanone (CP), pyrrole (PYRR), dimethyl sulfoxide (DMSO), acetone (ACE), phenol (PHEN), ethanol (EtOH), and methyl acetate (MEAC).